1. Field of the Invention
The present invention relates to redundant systems, and more particularly to redundant systems in the peripheral circuit to be used for liquid crystal displays, contact type image sensors, liquid crystal optical shutters and the like.
2. Description of the Related Art
For the purposes of reducing the size and cost as well as increasing the reliability of the product, there is known a technique in which thin film driver circuits are manufactured integrally with a liquid crystal display, a contact type image sensor, a liquid crystal optical shutter or the like. This technique is introduced based on the concept that it should be possible to drastically cut back the number of connection terminals and the number of external driver integrated circuits, and to resolve the problem of reliability that is generated due to the bonding process of large area and high density, by the installation of peripheral driver circuits on the same substrate as that of the pixel electrodes.
A shift register constitutes an important element of the thin film peripheral driver circuit as a serial-parallel converter. FIG. 1 is a diagram showing the conventional N-stage thin film shift register 103. Shift register stages 101 are able by means of a two-or four-phase clock to sequentially transfer a signal input to an input terminal 104 with a predetermined frequency from the first stage output to the N-th stage output, and are utilized as scanning circuits or a serial-parallel converter for picture signals.
Further, one-dimensional switching arrays are used, for example, at the time of employing a block driving system which is effective for apparently increasing the writing speed of picture signals.
FIG. 2 is a diagram showing the conventional one-dimensional switching array. The gate electrodes of switching transistors 301, of which each constitutes the switching array within one block, are connected to a common switching control line 303, and independent picture signals can be transferred, according to a block pulse input to the switching control line 303, from the drain side to the source side of the transistors 301 by means of the same timing switch.
FIG. 3 is a diagram showing a conventional example of the buffer circuit which serves as the interface between a shift register or a decoder and a part to be driven. A buffer 501 is a circuit which is required for driving a high capacitive load of the order of 100 pF.
The aforementioned peripheral driver circuit is ordinarily formed not by integrating amorphous silicon (referred to as a-Si hereinafter) thin film transistors but by polycrystalline silicon (referred to as poly-Si hereinafter) thin-film transistors. The reason for doing so is that the mobility of a poly-Si thin film transistor is larger by about two orders of magnitude than that of an a-Si thin film transistor permitting to expect peripheral driver circuits for higher driving frequencies, so that it become possible to deal with liquid crystal displays, contact type image sensors, liquid crystal optical shutters or the like with larger area and higher resolution.
With the increase in the dimension and the area of liquid crystal displays, contact type image sensors, liquid crystal optical shutters and the like, it is very difficult under the currently available processing technology to form defect-free thin film driver circuits. A defect in the shift register appears, for example, as a plane defect in a two-dimensional image of the liquid crystal display and the like. Further, a defect in a switching array or a buffer circuit appears as a line defect. Since these defects can occur even when the pixel array part is free from the defects, a poor yield of the peripheral driver circuits is becoming a substantial factor for deteriorating the yield for system as a whole of liquid crystal displays, contact type image sensors, liquid crystal optical shutters and the like.
In order to resolve such a problem, a configuration is disclosed in which a 480-bit regular shift register, for example, is made into one block and another 480-bit shift register is redundantly as its spare, in Y. Matsueda et al., "Defect-Free Active-Matrix LCD with Redundant Poly-Si TFT Circuit", SID 89 Dig est, pp. 238-241 (1989). However, with the above-mentioned shift register configuration having the 480-bit shift registers that form the regular and the spare blocks the yield was hardly improved.